This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting swirl to the measured fluid stream and a torque balance reaction generator for removing the imparted swirl. More particularly, this invention relates to such a meter having an improved readout system for indicating the mass rate of flow.
This invention is particularly adapted for use in a mass rate of flow meter which utilizes a spring-restrained turbine as the torque balance reaction generator. One such mass rate of flow meter is depicted in U.S. Pat. No. 4,056,976 issued Nov. 8, 1977 and titled Mass Rate of Flow Meter, which patent is assigned to the same assignee as the present invention. This meter includes a housing that defines a fluid passage that extends along a longitudinal axis through the housing and that has an input port and an output port located on the axis. A swirl generator is located adjacent the input port to impart a substantially constant angular velocity to an entering fluid stream. As the fluid leaves the swirl generator, it passes through an axially displaced, unrestrained rotor that rotates about the axis. The angular velocity of the rotor accurately represents the angular velocity of the fluid stream as it leaves the rotor and passes through an axially spaced, spring-restrained turbine. The angular momentum of the fluid stream angularly displaces the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is proportional to the mass rate of flow.
In a spring-restrained flowmeter, the rotor carries two circumferentially and longitudinally displaced bar magnets. The first magnet is disposed on the input end of the rotor and is circumferentially poled. A first sensing coil assembly in a transverse plane through the first magnet is radially spaced from the magnet and isolated from the fluid flow. Each time the first magnet passes the first sensing coil, it induces a "start" voltage pulse in the coil that indicates the passage of a predetermined point on the rotor past a predetermined point on the housing.
The second magnet is at the exit end of the rotor and diametrically opposed to the first magnet. An axially disposed, longitudinally extending bar of a highly permeable material, such as soft iron, mounts on the periphery of the turbine. The axial spacing between the rotor and the turbine interposes an axial air gap between the bar and the second magnet when they align. A second sensing coil assembly, that is isolated from the fuel flow, is coaxial with and longitudinally coextensive with the second magnet and the bar. Each time the second magnet passes the bar, the flux that the bar couples to the second sensing coil assembly changes and induces a "stop" voltage pulse in the second sensing coil. As described in the foregoing U.S. Pat. No. 4,056,976, timing circuits convert the start and stop pulses from the first and second sensing coil assemblies into an indication of the mass rate of flow through the meter.
The longitudinally extending permeable bar on the periphery of the turbine forms part of a magnetic path for the flux from the second magnet that is to link the second coil. The other parts of the magnetic path include magnetic shields around the second sensing coil. There is a substantial air gap in this magnetic path so that the overall reluctance of the magnetic path is relatively high. In fact, only a small percentage of the total flux from the second magnet is coupled through this magnetic path. The remaining flux leaks from the permeable bar to an air path that does not link the coil. With only a small percentage of the available flux being linked to the sending coil, the amplitude of the stop pulses is only a small percentage of the potential maximum amplitude. The resulting low signal-to-noise ratio can become troublesome, especially at low rotor speeds when the rate of flux change is reduced thereby further reducing the potential maximum amplitude. Under some conditions, therefore, the stop pulses can reach undetectible levels.